Depression is a pervasive psychiatric disease with significant associated personal, economic and medical costs. Despite evidence that the pathophysiology of depression differs for men and women, the mechanisms underlying these sex differences are unknown. This study will investigate homeostatic regulation of slow-wave activity (SWA) in EEG at baseline and in response to a 3-hour sleep delay in symptomatic depressed men and women and age- and gender-matched healthy controls. The long- range objective of this work is to determine the independent and interactive influence of sex and disease on sleep regulation and to understand the biological basis for sex differences in the pathophysiology, symptoms and relative risks for depression. The primary specific aims are: 1) To evaluate sex differences in the regulation of slow-wave activity (SWA) during sleep in response to a 3 hour sleep delay in 80 depressed men and women (n=40/group), 20-40 years of age. 2) To evaluate sex differences in the regulation of SWA in response to a 3 hour sleep delay in 80 healthy men and women (n=40/group), 20-40 years of age. 3) To confirm that the magnitude of sex differences in the regulation of SWA is larger in depressed patients than in healthy adults i.e. the group by sex interaction. The secondary aims include:4) To begin to establish the initial age-related changes in the regulation of SWA and to determine if the maturational time course differs by sex and disease. 5) To explore sex- and disease-dependent influences on the relationship between objective sleep physiology and subjective sleep characteristics. It is hypothesized that both depressed men and women will show homeostatic abnormalities, but it will be hyper-responsivity in women and hypo-responsivity in men. It is suggested that both conditions arise from sex-dependent propensities toward organizational abnormality observable only under challenge conditions in healthy individuals.